A distance measuring apparatus such as a current survey instrument irradiates an object of distance measurement with a measuring wave such as a laser beam or a microwave, detects a reflected wave (hereinafter, collectively called a measuring wave) reflected from the object of distance measurement, finds the distance of the reciprocating movement of the measuring wave according to the time lag between the emission and the detection of the measuring wave, and thereby calculates the distance (one-way distance) to the object.
Here, the measuring of the time lag (time interval) is performed, for example: by generating a high frequency clock signal having a known extremely-short cycle as compared with the time lag between the emission and the detection of a measuring wave; by counting the number of clocks in the high frequency clock signal generated during the time between the emission and the detection of the measuring wave; and then by multiplying the counted value by the cycle. However, even though the frequency in the clock signal has to be increased to improve the measurement accuracy in such measuring, there is a limitation on the increase of the generated frequency in the clock signal.
To address this issue, a method has been invented for making the frequency of a clock signal higher spuriously by generating multiple high-frequency clock signals at the same frequency which are a certain degree out of phase with each other, and by counting the number of clocks in each of the clock signals. However, in order to improve the reliability in accuracy, the measurement needs to be repeated the number of times corresponding to the number of generated clock signals. This leads to an increase of the measurement time, and hence to a problem in a practical use.
In order to solve this problem, the applicants have proposed a technique (Patent Document 1) in which: both a start signal synchronous with an emission time of a measuring wave and a stop signal synchronous with a detection time thereof are repeatedly generated two or more times at certain time intervals; a reference signal, such as a sinusoidal signal, generated in shorter cycle than the cycle of this repetition is sampled in response to the repeated start signals and in response to the repeated stop signals; a phase difference between the first sampling wave obtained with the start signal and the second sampling wave obtained with the stop signal; a phase difference between the start signal and the stop signal is found based on the phase difference between the sampling waves; and the obtained phase difference is converted into the time lag.
The foregoing proposed technique, however, requires the start signal and the stop signal to be generated two or more times, and then to be detected two or more times. For this reason, the proposed technique has a disadvantage in terms of further speeding-up of the measurement.
Moreover, in this proposed technique, multiple start signals and multiple stop signals individually operate as sampling signals. For this reason, this proposed technique requires a generation interval between the start signals and a generation interval between the stop signals to be accurately constant, and thereby needs the control for keeping the generation intervals constant.
Against this background, the applicants have proposed a technique for solving these problems. Specifically, for example, the applicants have proposed a time lag measuring device (Patent Document 2) in which: a start signal and a stop signal are waited to be generated under the condition where two reference signals in a known cycle, such as a sinusoidal signal and a cosine wave signal, having a phase difference of π/2 [rad] therebetween are generated; the amplitudes of the respective reference signals are detected by sampling both reference signals at each generation timing for the start signal and the stop signal; a phase at the detection time of the start signal is obtained based on the amplitudes of both reference signals obtained with the start signal; a phase at the detection time of the stop signal is obtained based on the amplitudes of both reference signals obtained with the stop signal; and the generation time lag between the start signal and the stop signal is obtained based on the known cycle of these reference signals and the difference (phase difference) between the phase at the detection time of the start signal and the phase at the detection time of the stop signal.
With this time lag measuring device, an accurate generation time lag between a start signal and a stop signal can be obtained by detecting a set of the start signal and the stop signal only once.
More precisely, for example, as shown in FIG. 7, a sinusoidal signal and a cosine wave signal (signal obtained by delaying the sinusoidal signal by a phase of π/2 [rad]) are generated as two reference signals, and both reference signals are sampled at timings of the start signal and the stop signal. Then, obtained are amplitudes A11 (an amplitude of the sinusoidal signal at the generation timing of the start signal), A12 (an amplitude of the cosine wave signal at the generation timing of the start signal), A21 (an amplitude of the sinusoidal signal at the generation timing of the stop signal), and A22 (an amplitude of the cosine wave signal at the generation timing of the stop signal) of both of the reference signals at both timings. By drawing, in an xy plane, the amplitude A12 of the cosine wave signal and the amplitude A11 of the sinusoidal signal corresponding to the generation timing of the start signal, an intersection Pstart is obtained as shown in FIG. 8A. Thus, arctan (A11/A12) (=tan−1 (A11/A12)) that is an arctangent value of a ratio between these amplitudes (A11/A12) indicates a phase shift amount _start (=tan−1 (A11/A12)) from a phase zero timing of the sinusoidal signal (or the cosine wave signal).
Similarly, by drawing, in an xy plane, the amplitude A22 of the cosine wave signal and the amplitude A21 of the sinusoidal signal corresponding to the generation timing of the stop signal, an intersection Pstop is obtained as shown in FIG. 8B. Thus, arctan (A21/A22) that is an arctangent value of a ratio between these amplitudes (A21/A22) indicates a phase shift amount _stop (=tan−1 (A21/A22)) from a phase zero timing of the sinusoidal signal (or the cosine wave signal).
Accordingly, a phase difference Δ_ in the reference signal between the generation timing of the start signal and the generation timing of the stop signal is obtained fromΔ_=_stop−_start,and a time lag Δt between the generation timing of the start signal and the generation timing of the stop signal is obtained fromΔt=(Δ—/2π)Ts [seconds],where Ts [seconds] denotes a cycle of the reference signal.
In addition, the applicants have also proposed a technique (Patent Document 3) in which: only a single reference signal is generated instead of the aforementioned two reference signals having the phase difference of π/2 [rad] therebetween; and, in the sampling of this reference signal by use of a pulse signal, the reference signal is sampled at two timings, i.e., a generation timing of the pulse signal and a delay timing that is delayed from this generation timing by the phase difference of π/2 [rad] between the reference signals.
In essence, the technique proposed by Patent Document 2 is that the two reference signals set to have a phase difference of π/2 [rad] are simultaneously sampled at one timing, while the technique proposed by Patent Document 3 is that the amplitude of the reference signal is sampled at the two timings shifted by a time corresponding to a phase difference of π/2 [rad] between the reference signals. The technique of Patent Document 2 and the technique of Patent Document 3 are substantially the same as that in which the amplitude of a reference signal is sampled at two phase difference timings corresponding to a phase difference of approximately π/2 [rad].    Patent Document 1: Japanese Patent Publication No. 2916780    Patent Document 2: Japanese Patent Application No. 2004-291495 (not disclosed)    Patent Document 3: Japanese Patent Application No. 2005-169500 (not disclosed)